1. Field of the Invention
The present invention relates to a vehicle differential clutch and more particularly, to a differential clutch where friction discs are selectively engaged using a ball ramp actuator where a one-way clutch is used to provide efficient and effective clutch activation.
2. Description of Related Art
Differential clutches commonly use a series of clutch plates actuated by manual, hydraulic or electrical systems to provide various conditions of open differentials, limited slip differentials or locked-up differentials.
Efforts to automate the operation of the clutch using electronics are currently underway. It is known to use an electromechanical or hydraulic actuator connected to the mechanical linkage to, in essence, replace the operator for more accurate clutch operation during slippery conditions. Using such an actuator, the mechanical linkage is moved in response to an electrical control signal generated by a central microprocessor used to process a variety of vehicle sensor inputs and other operating conditions to determine when and in what manner the differential clutch should be activated, or deactivated.
The use of a ball ramp actuator to load a clutch pack in a vehicle driveline differential is known from the prior art, including limited slip differentials where a clutch pack is loaded in response to the activation of a ball ramp actuator initiated by rotation of a servo motor or a solenoid driven brake shoe on an activating ring. The advantage of the ball ramp mechanism over other actuators is that it converts rotary motion into axial motion with very high force amplification, often 100:1 or greater. A ball ramp actuator has also been utilized in a vehicle transmission to engage and disengage gearsets by loading a gear clutch pack in response to a signal.
In both of these applications, one side of the ball ramp actuator, commonly called a control ring, reacts against case ground through the force induced by an electromagnetic field generated by a coil or is rotated by an electric motor relative to case ground. To generate greater clamping forces, the electrical current supplied to the coil or motor is increased thereby increasing the reaction of the control ring to case ground which rotates the control ring relative to an activation ring thereby causing rolling elements to engage ramps in the control and activation ring which increase the axial movement and clamping force on the clutch pack.
The ball ramp actuator comprises a plurality of roller elements, a control ring and an opposed activation ring where the activation ring and the control ring define at least three opposed single ramp surfaces formed as circumferential semi-circular grooves, each pair of opposed grooves containing one roller element. A plurality of thrust balls (or other type of thrust bearing) is interposed between the control ring and a housing member, rotating with and connected to the input member such as a flywheel. An electromagnetic coil is disposed adjacent to one element of a control clutch so as to induce a magnetic field that loads the control clutch which in turn applies a force on the control ring of the ball ramp actuator.
One problem with the use of a ball ramp actuator to supply the clutch clamping force is that the mechanics of prior art unidirectional ball ramp mechanisms result in a limited sensitivity due the short travel distance of the ball and ramp assembly due to limited space. Providing ramps at a constant radius or concentric cam path provides a limited travel path for the ball(s) and thereby limits sensitivity. In addition, the conventional ball ramp actuator uses a cam ramp that operates in only one direction (single ramp). However, a vehicle that is stuck may move forward and reverse to free itself, and torque reversal on the drive shaft and axle occurs and is expected in those situations. The single ramp arrangement cannot effectively transmit torque in both the forward and reverse directions without a one-way clutch or constant electric signal to maintain engagement.
In order for the single ramp arrangement to work, thrust bearings must be disposed on both sides of the 3-piece cam plate assembly that is nonrotatably attached to the differential, and this arrangement increases the expense.
The need exists for a differential clutch actuation system having improved sensitivity and improved cam ramp axial travel versus cam ramp angle while at the same time providing both forward and reverse actuation.
The invention provides for a ball ramp actuator for an electronically controlled clutch such as might be used in a motor vehicle, wherein a ball ramp actuator has forward and reverse angle ramps and improved sensitivity and cam ramp axial travel versus cam ramp angle.
The present invention is characterized by a differential clutch assembly coupled through a three-piece cam plate construction with dual-direction, overlapping ramps that allow twice the cam ramp travel distance while maintaining the cam angle. An electromagnetic coil is used to activate a control clutch, which frictionally couples the clutch discs. The ball ramp actuator provides a clamping force on the clutch friction discs whose amplitude immediately increases with the differential speed between the input and output shafts without complex electronic intervention using the coil.
A provision of the present invention is the use of a bi-directional cam plate construction that permits a vehicle that is stuck or slipping to move forward and reverse to obtain positive traction. In other words, the present invention operates when the torque reversal on the drive shaft and axle occurs.
Another provision of the present invention is to provide an interaxle application where input rotation can be clockwise or counterclockwise and there is limited space for a mechanism to engage the ball ramp.
Still another provision of the present invention is the elimination of one-way clutches, thrust bearings and gear reduction assemblies.
These and other benefits of the present invention will become more apparent with reference to the following drawings and associated description.